Studying the other users problems and the SFCFIX.zip that forum admins proposed to them, I've been able to fix my system:
Looking at my CBS.log it is clear (lines 6252-6270) that a number of cat/mum files are missing:
[...]
missing package manifest/cat for package:Containers-ApplicationGuard-Shared-merged-Package~31bf3856ad364e35~amd64~~10.0.19041.3393
[...]
missing package manifest/cat for package:Multimedia-RestrictedCodecsCore-WCOSMinusHeadless-Package~31bf3856ad364e35~amd64~~10.0.19041.3393
[...]
All of them end with 10.0.19041.3393, and googling "10.0.19041.3393" it turns out that these OS files came with KB5029331, so:
1. I downloaded KB5029331 from
www.catalog.update.microsoft.com
2. I uncompressed the downloaded .msu file. Even Microsoft
suggests to do this
3. I copied all .cat and .mum files that CBS.log mentions to a temp directory
4. In this temp directory I added a file SFCFIX.txt with the following content:
::
{ARCHIVE} %SystemRoot%\servicing\packages [DIR]
5. zipped this temp dir into a SFCFIX.zip file and dropped the zip onto SFCFix.exe
6. I run in an administator command prompt:
DISM /ONLINE /CLEANUP-IMAGE /SCANHEALTH
DISM /ONLINE /CLEANUP-IMAGE /CHECKHEALTH
DISM /ONLINE /CLEANUP-IMAGE /RESTOREHEALTH
The error is gone!